Fracture hazards in equipment such as high energy piping complexes and components may arise from flaw development processes such as electro-chemical corrosion, corrosion product nucleation and development, pitting, plastic deformation development, and micro-cracking resulting, for example, from corrosion-fatigue. These flaw development processes may accelerate fracture development, particularly in the case of a material subject to joint action of corrosion and cyclic loading. As a result, a failure may occur at substantially lower loads and/or after a shorter time than would be predicted in the absence of the flaw development processes.
Fracture hazard risk is influenced by equipment design and operating regime. For example, corrosion-fatigue of steel piping may be influenced by interaction with water or steam chemistry and/or oxygen, and the presence of stress concentrators. In addition, multiple temperature cycles in certain environments may result in oxide cracking, crack formation and development in the steel under the cracked scale layer, and, consequently, acceleration of material property degradation. As a further example, high speed steam flow carrying scale particles may cause erosion and accelerate oxide film delaminating and spoliation.
Non-destructive inspection (NDI) methods of conventional types are inadequate to determine whether operating structures have suffered hazardous damage due to such flaw development processes. In particular, for example, known methods are impractical for overall inspection of micro- and macro-flaws in complex and large-scale high energy piping complexes and similar equipment. As a result, catastrophic failures of, for example, chemical and fossil fuel power plants' main steam piping, and hot and cold reheat piping, chemical reactors' piping, and technological piping of chemical plants are not uncommon. Such failures present a serious economic and human safety hazard. In many instances, non-destructive and destructive evaluation of failed components demonstrated conclusively that the cause of failure was related to electro-chemical corrosion, pitting, plastic deformation development, and corrosion-fatigue micro-cracking. Conventional NDI techniques, unfortunately, are incapable of revealing, locating, identifying and assessing individual and interacting flaws with low and high stress intensity according to fracture mechanics criteria in equipment such as operating high energy piping complexes and components. As a result, structural elements that exhibit no signs of significant damage detectable by conventional methods may nevertheless represent future—even imminent—hazards. Thus, there is a significant unmet need for techniques to identify such propagating flaw development processes in time to mitigate the hazard and prevent catastrophic failure.
The present inventors have developed improved techniques for revealing flaws in reinforced concrete structures and for the diagnosis of creep in high energy piping during operation. QAE NDI is based on the fact that structures and materials undergoing deformation and flaw-development processes emit acoustic emissions (AE), in particular, sound and ultrasound waves. These AE, after acquisition by piezoelectric sensors, for example, may be digitized and analyzed. The analysis can reveal the location of flaws, their type and danger level. Since flaw development occurs under load, the consequent acoustic emission signals may be acquired during operation, pressure test, or cool down of the high energy piping. Muravin, G., “Inspection, Diagnostics and Monitoring of Construction Materials and Structures by the Acoustic Emission Method”, 2000, Minerva Press, London, 480 pages, the disclosure of which is hereby incorporated in its entirety into the present application.
A number of known acoustic emission NDI techniques are disclosed by Coulter, et al., U.S. Pat. No. 5,526,689 (hereinafter, “Coulter”) and Brock, et al., U.S. Pat. No. 6,823,736 (hereinafter, “Brock”). As identified by Brock, for example, “active” acoustic emission NDI systems employ an external excitation source (e.g., an ultrasonic probe signal) to impinge a sample under study. In such an active system, an acoustic transducer receives an acoustic emission signal from the test sample that resulted from perturbation of the original probe signal due to interfaces, structural changes or defects in the material. Disadvantages of NDI systems that use such active systems include complexity in quantitative analysis due to the need to decouple parasitic acoustic signals between the source and the detectors, and risk that the external excitation source signal may cause or accelerate damage to the sample under study. Brock further discloses known “passive” systems which use an acoustic detector but no acoustic or other external excitation source. According to Brock, such known passive systems suffer several disadvantages: (i) they may only be used to estimate the amount of damage in the material or how long a component will last; (ii) signals are typically smaller because there is no means to increase the level of the stimuli to increase signals; and (iii) because service environments are very noisy, and acoustic emission signals tend to be very weak, signal discrimination and noise reduction are difficult. Brock, 1:7-34.
For the foregoing reasons, and because the energy of acoustic emission signals associated with flaw development processes such as corrosion product nucleation and development, pitting, plastic deformation development, and micro-cracking is very low, passive acoustic emission NDI techniques have not been previously used for detection of such processes, notwithstanding the potential advantages of such methods with respect to active acoustic emission NDI techniques.